Modulating apparatus, mobile communication system, modulating method, and communication method

ABSTRACT

An encoder encodes sound data and the like to generate a binary signal. A mapper converts the binary signal into a quaternary symbol and outputs the quaternary symbol. A base band filter includes a root raised cosine filter and a sinc filter. The base band filter blocks a predetermined frequency component of a symbol to shape the symbol into a waveform signal and outputs the waveform signal shaped. An FM modulator transmits a signal subjected to FM modulation according to a magnitude of an amplitude of a waveform signal to a receiving unit. When a symbol of ±3 is outputted from the mapper, a frequency shift of a signal transmitted from the FM modulator has a predetermined value in a range of ±0.822[kHz] to ±0.952[kHz]. This makes it possible to provide a modulating apparatus, a mobile communication system, a modulating method, and a communication method that use a modulating method that can conform to the FCC rule to be enforced in 2005 without using a linear power amplifier.

TECHNICAL FIELD

The present invention relates to a modulating apparatus, a mobilecommunication system, a modulating method, and a communication method,and more particularly, to a modulating apparatus, a mobile communicationsystem, a modulating method, and a communication method that can conformto the FCC rule to be enforced in 2005 without using a linear poweramplifier.

BACKGROUND ART

Conventionally, the Land Mobile Radio (LMR) system in the United Stateshas extremely low spectrum efficiency because frequencies are allocatedto respective channels at extremely wide channel spacing such as 25 kHzor 30 kHz. To improve this spectrum efficiency, the FederalCommunications Commission (FCC) of the United States provided in the FCCrule that, from 1997, the channel spacing be changed to 12.5 kHz thatwas half the conventional spectrum efficiency. Consequently, the LMRsystem was operated at the channel spacing of 25 kHz or 12.5 kHz. Thischange of the channel spacing is explained in the FCC Rule (Title 47Code of Federal Regulations PART 90: Private Land Mobile RadioServices)).

To further improve the spectrum efficiency, in 2005, the FCC rule forchanging the channel spacing to 6.25 kHz, which is half the presentspectrum efficiency, will be enforced. Therefore, in accordance with theenforcement of the FCC rule in 2005, it is necessary to develop an LMRsystem that can be operated even at the channel spacing of 6.25 kHz.

The FCC rule to be enforced in 2005 provides, as conditions for the LMRsystem, that the LMR system have spectrum efficiency capable ofoperating one sound channel per 6.25 kHz band and have a transfer rateequal to or higher than 4800 bps per 6.25 kHz band. It is possible toallow the LMR system to operate one sound channel per 6.25 kHz band byadopting the FDMA (Frequency Division Multiple Access) system in the6.25 kHz band or the 4-slot TDMA (Time Division Multiple Access) systemin the 25 kHz band. In recent years, since it is a general practice totransmit character data and the like in addition to sound data, it isdesirable that the LRM system to be developed for the enforcement of theFCC rule in 2005 is capable of communicating not only sound data butalso character data and the like.

First, the FCC rule that sets the standard of the present LMR systemapplicable to the channel spacing of 12.5 kHz will be explained. The FCCrule (Title 47 Part 90.210: Emission masks) prescribes emission maskscorresponding to respective bands. A mask D having characteristics shownin Table 1 below and FIG. 1 is stipulated for the 12.5 kHz band. TABLE 1Displacement Frequency Range Attenuation (dB) fd < 5.625 kHz 0 5.625 kHz< fd < 12.5 kHz 7.27 (fd − 2.88) fd > 12.5 kHz 70 or 50 + 10 log10(P)whichever the lesser ATT

In the table, fd is a displacement frequency range from a centerfrequency and is represented by a unit kHz. P is a transmission powerand is represented by a unit W.

Most of the present LMR systems applicable to the channel spacing of12.5 kHz transmit an analog sound signal after subjecting the analogsound signal to Frequency Modulation (FM modulation) (analog FMmodulation). A sound band and a maximum frequency shift of the LMRsystems are as shown in Table 2 below. TABLE 2 Modulation System FMModulation Sound Band 0.3 kHz to 3 kHz Maximum Frequency Shift ±2.5 kHzShift

The modulation system (APCO P25 Phase 1 modulation system, hereinafter“P25-P1 modulation system”), which was examined as the Project 25 in theAPCO (Association of Public Safety Communications Officials) and, then,enacted as the standard (TIA102) by the TIA (Telecommunications IndustryAssociation) is also used as a modulation system of the LMR systemapplicable to the channel spacing of 12.5 kHz. This modulation system isa system for transmitting a digital signal of a base band according toquaternary FSK modulation. A transmission rate, a symbol rate, a baseband filter, and a nominal frequency shift of the modulation system areas shown in Table 3 below. TABLE 3 Transfer Rate 9600 bps Symbol Rate4800 symbol/s Base Band Filter Transmission: A filter obtained bycombining a filter having a Raised Cosine characteristic with α = 0.2and a shaping filter Reception: An integrate and dump filter ModulationSystem Quaternary FSK Modulation Nominal Frequency Shifts of +3 = +1.8kHz, +1 = +0.6 kHz, Shift −1 = −0.6 kHz, and −3 = −1.8 kHz forrespective four symbol levels (±3, ±1)

In measuring an emission spectrum at the time when the analog FMmodulation system is used, the FCC rule provides, as a measurementcondition, that a modulation frequency be set to 2.5 kHz and that theemission spectrum be measured by being modulated at a level increased by16 dB from a modulation signal level at which 50% of a maximum frequencyshift is obtained. A waveform of the emission spectrum set to satisfythis condition and the emission mask (mask D) are shown in FIG. 2. Asshown in FIG. 2, it is provided that the emission mask be substantiallyat the same level as a line of a high-order component of 2.5 kHz.

A waveform of an emission spectrum measured using pseudo-random data asa modulation signal in the P25-P1 modulation system and the emissionmask (mask D) are shown in FIG. 3. Since the pseudo-random data is usedas the modulation signal, as shown in FIG. 3, the emission spectrummeasured has a uniformly distributed spectrum shape and conforms to themask D.

A waveform of an emission spectrum measured using data with symbols +3and −3, that is, data with shifts +1.8 kHz and −1.8 kHz rather than thepseudo-random data and the emission mask (mask D) are shown in FIG. 4.Since a symbol rate is 4800. symbol/s and a rectangular wave is shapedto a sine wave by a base band filter, the emission spectrum measured isequivalent to a spectrum subjected to FM modulation with a frequencyshift of a certain value by a sine wave of 2.4 kHz. As shown in FIG. 4,the emission spectrum measured has a peak at a frequency integer timesas high as 2.4 kHz and third-order and fourth-order components of theemission spectrum slightly deviate from the mask D.

The FCC rule stipulates a mask E having a characteristic shown in Table4 below and FIG. 5 for the 6.25 kHz band. TABLE 4 Displacement FrequencyRange Attenuation (dB) fd < 3.0 kHz 0 3.0 kHz < fd < 4.6 kHz 65 or 30 +16.67(fd − 3) or 55 + 10 log10(P) whichever the lesser ATT fd > 4.6 kHz65 or 55 + 10 log10(P) whichever the lesser ATT

In order to adapt an emission spectrum to the mask E, the analogmodulation FM modulation system is applied as the modulation system inone case and the P25-P1 modulation system is applied as the modulationsystem in another case. These cases will be examined.

In the past, when the channel spacing was revised from 25 kHz to 12.5kHz, an emission spectrum could be adapted to the emission mask (mask D)stipulated by the FCC rule by changing a frequency shift in the analogmodulation FM modulation system from the 5 kHz shift to the 2.5 kHzshift. A waveform of an emission spectrum measured by changing afrequency shift from the 2.5 kHz shift to the 1.25 kHz shift followingthis example (at a modulation frequency of 2.5 kHz) and the emissionmask (mask E) are shown in FIG. 6. As shown in FIG. 6, obviously, theemission spectrum measured does not conform to the emission mask.

A waveform in the case in which a transmission rate and a frequencyshift were set to half as large as those at a channel spacing of 12.5kHz and pseudo-random data was used as a modulation signal in the P25-P1modulation system and the emission mask (mask E) are shown in FIG. 7. Awaveform of an emission spectrum in the case in which data with a symbolalternately taking ±3 was used as a modulation signal in the P25-P1modulation system and the emission mask (mask E) are shown in FIG. 8.

Since the emission spectrum shown in FIG. 7 is uniformly distributed,the emission spectrum seemingly conforms to the emission mask. However,actually, as shown in FIG. 8, the emission spectrum does not conform tothe emission mask.

Simply by halving the parameters such as a transmission rate and afrequency shift of the modulation system currently applied to the LMRsystem applicable to the channel spacing of 12.5 kHz in this way, it isimpossible to adapt the emission spectrum to the emission mask (mask E).

A case in which another modulation system is used for the LMR system toadapt an emission spectrum to an emission mask (mask E) will beexplained. For example, a case in which a modulation system of the APCOP25 Phase 2 standard (hereinafter, “P25-P2 modulation system”) isapplied to the LMR system will be examined. This P25-P2 modulationsystem is a system in which a modulation system on a transmission sideis changed to the π/4QPSK modulation system while keeping a data formatof the P25-P1 modulation system as it is. A transmission rate, a symbolrate, a base band filter, and a phase shift of this modulation systemare as shown in Table 5 below. TABLE 5 Transmission Rate 9600 bps SymbolRate 4800 symbol/s Base Band Filter Transmission: A filter having aRaised Cosine characteristic with α = 0.2 Reception: An integrate anddump filter Modulation System π/4QPSK modulation Phase Shift Shifts of+3 = +3/4p, +1 = +1/4p, −1 = −1/4p, and −3 = −3/4p for respective foursymbol levels (±3, ±1)

A waveform of an emission spectrum measured when pseudo-random data wasmodulated by the P25-P2 modulation system and the emission mask (mask E)are shown in FIG. 9. Since the P25-P2 modulation system is based on theπ/4QPSK modulation system, as shown in FIG. 9, the emission spectrummeasured has a characteristic of steeply attenuating out of a band andconforms to the emission mask (mask E) despite the fact that thetransmission rate is 9600 bps.

However, since the π/4QPSK modulation system is a linear modulationsystem, problems described below occur.

It is impossible to use a nonlinear power amplifier used in the presentLMR system. In order to use a linear power amplifier in the LMR system,since an additional circuit such as a linearizer is required, a spaceand cost for the LMR system increase. Since the linear power amplifierhas lower efficiency and a larger consumed current compared with thenonlinear power amplifier, heat generation in a radio apparatusconstituting the LMR system causes a problem. Moreover, in a portableLMR system, since the portable LMR system is driven by a battery, anoperation time decreases.

At this moment, a linear power amplifier having an output power that isthe same as that of the conventional nonlinear power amplifier andhaving efficiency equivalent to that of the conventional nonlinear poweramplifier has not been developed. In addition, it is extremely difficultto make a mounting space and cost of a nonlinear power amplifierequivalent to those of the non-linear power amplifier. Therefore, it isnot realistic to apply the linear modulation system represented by theP25-P2 modulation system to the LMR system applicable to the channelspacing of 6.25 kHz.

The invention has been devised to solve the problems and it is an objectof the invention to provide a modulating apparatus, a mobilecommunication system, a modulating method, and a communication methodthat can conform to the FCC rule to be enforced in 2005 without using alinear power amplifier.

DISCLOSURE OF THE INVENTION

In order to attain the object of the invention, the invention provides amodulating apparatus in a mobile communication system that performs datacommunication at a rate for transmitting 2400 multi-value symbols persecond. This modulating apparatus includes: a base band filter thatblocks an unnecessary frequency component of a multi-value symbolinputted and outputs a waveform signal; and frequency shifting andmodulating means for shifting to modulate a frequency of an outputsignal according to a magnitude of an amplitude of the waveform signalinputted from the base band filter. The frequency shifting andmodulating means is adjusted such that, when a symbol having a maximumabsolute value is inputted, an output signal has an absolute value of afrequency shift in a range of 0.822[kHz] to 0.952[kHz].

In order to attain the object, a modulating apparatus according to afirst aspect of the invention is a modulating apparatus in a mobilecommunication system that performs data communication at a transmissionrate of 2400×(n+1) (n: natural number) [bps]. The modulating apparatusincludes: symbol converting means for sequentially converting a binarysignal generated by encoding predetermined data into a 2^((n+1))-arysymbol, which includes (2^((n+1))+1−2k) (1≦k≦2^((n+1))) values, (n+1)bits at a time and outputting the symbol; a base band filter that blocksan unnecessary frequency component of a symbol inputted from the symbolconverting means and outputs a waveform signal; and frequency shiftingand modulating (FM) means for shifting to modulate a frequency of anoutput signal according to a magnitude of an amplitude of the waveformsignal inputted from the base band filter. When a symbol of±(2^((n+1))−1) is outputted from the symbol converting means, afrequency shift of the output signal from the frequency shifting andmodulating means is set to take a value in a range of ±0.822[kHz] to±0.952[kHz].

In the modulating apparatus, the base band filter may be a Nyquistfilter.

In order to attain the object, a mobile communication system accordingto a second aspect of the invention is a mobile communication systemincluding: a transmitter that performs transmission of data at atransmission rate of 2400×(n+1) (n: natural number) [bps]; and areceiver that receives data transmitted from the transmitter. Thetransmitter includes: encoding means for encoding predetermined data togenerate a binary signal; symbol converting means for sequentiallyconverting a binary signal generated by the encoding means into a2^((n+1))-ary symbol, which includes (2^((n+1))+1−2k) (1≦k≦2^((n+1)))values, (n+1) bits at a time and outputting the symbol; a first baseband filter that blocks an unnecessary frequency component of a symbolinputted from the symbol converting means and outputs a waveform signal;and frequency shifting and modulating (FM) means for transmitting asignal, which is obtained by shifting to modulate a frequency accordingto a magnitude of an amplitude of the waveform signal inputted from thefirst base band filter, to the receiver. The receiver includes:demodulating means for demodulating the signal transmitted from thetransmitter and received and outputting a 2^((n+1))-ary signal; a secondbase band filter that blocks an unnecessary frequency component of the2^((n+1))-ary signal outputted from the demodulating means and outputsthe 2^((n+1))-ary signal; binary signal converting means forsequentially converting a 2^((n+1))-ary signal inputted from the secondbase band filter into a binary signal of (n+1) bits and outputting thebinary signal; and decoding means for decoding a binary signal inputtedfrom the binary signal converting means and outputting the predetermineddata. When a symbol of ±(2^((n+1))−1) is outputted from the symbolconverting means, a frequency shift of a signal outputted from thefrequency shifting and modulating means is set to take a value in arange of ±0.822[kHz] to ±0.952[kHz].

In the mobile communication system, the first and second base bandfilters may be Nyquist filters.

In the mobile communication system, the first base band filter mayinclude a root raised cosine filter and a sinc filter. The second baseband filter includes a root raised cosine filter and a 1/sinc filterthat has a characteristic opposite to that of the sinc filter. A nominalfrequency shift of the symbol of ±(2^((n+1))−1) may be set to a valueπ/2√{square root over (2)} times as large as a frequency shift of asignal outputted from the frequency shifting and modulating means.

Moreover, in the mobile communication system, the first and second baseband filters may include root raised cosine filters. The nominalfrequency shift of the symbol of ±(2^((n+1))−1) may be set to a value1/√{square root over (2)} times as large as a frequency shift of asignal outputted from the frequency shifting and modulating means.

In the mobile communication system, the first base band filter mayinclude a raised cosine filter and a 1/sinc filter. The second base bandfilter may include a sinc filter that has a characteristic opposite tothat of the 1/sinc filter. The nominal frequency shift of the symbol of±(2^((n+1))−1) may be set to a value 2/π times as large as a frequencyshift of a signal outputted from the frequency shifting and modulatingmeans.

In order to attain the object, a modulating method according to a thirdaspect of the invention is a modulating apparatus in a mobilecommunication system that performs data communication at a transmissionrate of 2400×(n+1) (n: natural number) [bps]. The modulating methodincludes: a symbol converting step of sequentially converting a binarysignal generated by encoding predetermined data into a 2^((n+1))-arysymbol, which includes (2^((n+1))+1−2k) (1≦k≦2^((n+1)))values, (n+1)bits at a time and outputting the symbol; a step of blocking anunnecessary frequency component of a symbol inputted at the symbolconverting step and outputting a waveform signal; and a frequencyshifting and modulating (FM) step of shifting to modulate a frequency ofan output signal according to a magnitude of an amplitude of thewaveform signal inputted. When a symbol of ±(2^((n+1))−1) is outputtedfrom the symbol converting step, a frequency shift of the output signalfrom the frequency shifting and modulating step is set to take a valuein a range of ±0.822[kHz] to±0.952[kHz].

In order to attain the object, a communication method according to afourth aspect of the invention is a communication method in a mobilecommunication system including: a transmitter that performs transmissionof data at a transmission rate of 2400×(n+1) (n: natural number) [bps];and a receiver that receives data transmitted from the transmitter. Thecommunication method includes: an encoding step of encodingpredetermined data to generate a binary signal; a symbol converting stepof sequentially converting a binary signal generated by the encodingstep into a 2^((n+1))-ary symbol, which includes (2^((n+1))+1−2k)(1≦k≦2^((n+1))) values, (n+1) bits at a time and outputting the symbol;a step of blocking an unnecessary frequency component of a symbolinputted from the symbol converting step and outputting a waveformsignal; a frequency shifting and modulating (FM) step of transmitting asignal, which is obtained by shifting to modulate a frequency accordingto a magnitude of an amplitude of the waveform signal inputted from thefirst base band filter, to the receiver; a demodulating step ofdemodulating the signal transmitted from the transmitter and receivedand outputting a 2^((n+1))-ary signal; a step of blocking an unnecessaryfrequency component of the 2^((n+1))-ary signal outputted from thedemodulating step and outputting the 2^((n+1))-ary signal; a binarysignal converting step of sequentially converting a 2^((n+1))-ary signalinputted into a binary signal of (n+1) bits and outputting the binarysignal; and a decoding step of decoding a binary signal inputted fromthe binary signal generating step and outputting the predetermined data.When a symbol of ±(2^((n+1))−1) is outputted from the symbol convertingstep, a frequency shift of a signal outputted from the frequencyshifting and modulating step is set to take a value in a range of±0.822[kHz] to ±0.952[kHz].

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a characteristic of a mask D;

FIG. 2 is a graph showing a waveform of an emission spectrum in the casein which an analog FM modulation system is used and the mask. D;

FIG. 3 is a graph showing a waveform of an emission spectrum in the casein which pseudo-random data is converted in a P25-P1 modulation systemand the mask D;

FIG. 4 is a graph showing a waveform of an emission spectrum in the casein which symbols of ±3 are alternately generated in the P25-P1modulation system and the mask D;

FIG. 5 is a graph showing a characteristic of a mask E;

FIG. 6 is a graph showing a waveform of an emission spectrum in the casein which the analog FM modulation system is used and the mask E;

FIG. 7 is a graph showing a waveform of an emission spectrum in the casein which a transmission rate and a frequency shift in the P25-P1modulation system are halved and pseudo-random data is modulated and themask E;

FIG. 8 is a graph showing a waveform of an emission spectrum in the casein which a transmission rate and a frequency shift in the P25-P1modulation system are halved and symbols of ±3 are alternately generatedand the mask E;

FIG. 9 is a graph showing a waveform of an emission spectrum in the casein which pseudo-random data is modulated in a P25-P2 modulation systemand the mask E;

FIG. 10 is a graph showing an error rate characteristic in the case inwhich respective candidate filters are used;

FIG. 11 is a graph showing an error rate characteristic in the case inwhich respective candidate filters are used;

FIG. 12 is a block diagram showing a constitution of a ground mobilecommunication system according to a first embodiment of the invention;

FIG. 13 is a flowchart for explaining operations of the ground mobilecommunication system according to the first embodiment;

FIG. 14 is a graph showing a waveform of an emission spectrum in thecase in which symbols of ±3 are alternately generated in the groundmobile communication system according to the first embodiment and themask E;

FIG. 15 is a graph showing a waveform of an emission spectrum in thecase in which pseudo-random data is modulated in the ground mobilecommunication system according to the first embodiment and the mask E;

FIG. 16 is a block diagram showing a constitution of a ground mobilecommunication system according to a second embodiment of the invention;

FIG. 17 is a graph showing a waveform of an emission spectrum in thecase in which symbols of ±3 are alternately generated in the groundmobile communication system according to the second embodiment and themask E;

FIG. 18 is a graph showing a waveform of an emission spectrum in thecase in which pseudo-random data is modulated in the ground mobilecommunication system according to the second embodiment and the mask E;and

FIG. 19 is a block diagram showing a constitution of a ground mobilecommunication system according to a modification of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be hereinafter explained in detailwith reference to the drawings.

As described above, it is impossible to adapt an emission spectrum to anemission mask (mask E) simply by halving parameters such as atransmission rate and a frequency shift of a modulation system currentlyapplied to an LMR system applicable to a channel spacing of 12.5 kHz. Itis realistically difficult to use a linear modulation system representedby a P25-P2 modulation system because of cost and the like.

Thus, in the embodiments described below, parameters such as atransmission rate and a frequency shift that can realize an LMR systemconforming to the FCC rule to be enforced in 2005 by adopting anonlinear modulation system represented by a quaternary FSK modulationsystem will be examined.

It is known that a spectrum of an FM modulation wave at the time when amodulation signal is a sine wave is represented by a Bessel functionindicated by Expression (1) below.φFM(t)=A[J0(mf)cosωct+J1(mf){cos(ωc+ωm)t-cos(ωc-ωm)t}+J2(mf){cos(ωc+2ωm)t-cos(ωc-2ωm)t}+J3(mf){cos(ωc+3ωm)t-cos(ωc-3ωm)t}+.. . ]  (1)where, ωc is a frequency of a carrier wave, ωm is a modulated frequency,mf is a modulation index (a frequency shift/a modulation frequency), andJn(mf) is a Bessel function of the first kind for an nth ordercomponent.

Correlation between the emission spectrum shown in FIG. 2 and the Besselfunction indicated by Expression (1) is calculated. Since a modulationfrequency is 2.5 kHz and a frequency shift is a 2.5 kHz shift, themodulation index mf is 1. When primary to quaternary components arecalculated according to the Bessel function, the following levels areobtained.

(Primary)=J1(1)=−7.13 dB

(Secondary)=J2(1)=−18.79 dB

(Third-order)=J3(1)=−34.17 dB

(Fourth-order)=J4(1)=−52.12 dB

When these values and values of peak spectra generated at frequenciesinteger times as high as 2.5 kHz in FIG. 2 are compared, errors thereofare within a range of ±1 dB. Therefore, a mission spectrum of an FMmodulation wave using a sine wave for a modulation signal may becalculated by Expression (1).

A frequency shift conforming to this mask is calculated backward from avalue of the mask E stipulated in the FCC rule. A stipulated level ofthe mask E in the secondary component is calculated as −65 dB from FIG.5. A modulation index mf for setting an emission spectrum to be equal toor lower than this level is calculated as 0.067 by backward calculation.A frequency shift in this case is an extremely small value of0.067×2.5=0.167 kHz.

A general S/N (Signal/Noise) of an LMR system that is applicable to thechannel spacing of 12.5 kHz and uses the frequency shift of 2.5 kHz isabout 45 dB. When the frequency shift is halved, the S/N falls by 6 dB.Thus, an S/N at the time when a frequency shift is 0.167 kHz iscalculated as 45+20×log10 (0.167/2.5)=21.5 dB. This clearly indicatesperformance that cannot stand a practical use.

Consequently, although it is possible to adopt a frequency shift byreducing the frequency shift using the analog FM modulation, sinceperformance cannot be permitted in terms of a practical use, this methodis excluded from an object of examination.

A condition under which the LMR system adopting the quaternary FSKmodulation system conforms to the FCC rule to be enforced in 2005 willbe explained.

In FIGS. 3 and 4, an emission spectrum in the case in which the P25-P1modulation system is adopted is shown. In the case of digitalmodulation, a spectrum of a modulation wave is different depending on acharacteristic of a data sequence used. As is evident from FIGS. 3 and4, if random data is used, the spectrum disperses and average energy perunit frequency decreases. Thus, the spectrum is seemingly narrowed. Inrepetition of specific data, an emission spectrum is equivalent to aspectrum subjected to modulation with a sine wave. Since energyconcentrates on components integer times as large as the sine wave, awide spectrum is obtained. Therefore, a worst condition in thequaternary FSK modulation is a condition at the time when symbols of +3and −3 with a wider frequency shift are used and the symbols arealternately repeated to subject an emission spectrum to modulation witha sine wave equal to a frequency half as high as a symbol rate.

As described above, the FCC rule to be enforced in 2005 provides, as acondition, that an emission spectrum be adapted to the mask E and that,in performing transmission of data, an LMR system have a transmissionrate equal to or higher than 4800 bps per 6.25 kHz band. Thus, afrequency shift for adapting an emission spectrum to the mask E at thetime when the quaternary FSK modulation system is used at a transmissionrate of 4800 bps is calculated backward. In this case, since a symbolrate is 2400 symbol/s that is half the transmission rate of 4800 bps.Thus, by repeating symbols of +3 and −3, an emission spectrum isequivalently a sine wave of 1.2 kHz.

When a standard value of the mask E and a value of a spectrum integertimes as large as 1.2 kHz are compared, a fourth order is most strict interms of a condition. If this condition is satisfied, it is possible toadapt the emission spectrum to the mask E. In this case, the modulationindex mf is 0.685 and a frequency shift takes a value indicated byExpression (2) below.0.685×1.2=0.822 kHz   (2)

As described above, the emission spectrum shown in FIG. 4 does notconform to the mask D. The emission spectrum shown in FIG. 4 is measuredunder the worst condition in the P25-P1 modulation system. Followingthis example, an emission spectrum does not have to conform to anemission mask under the worst condition. In an actual state of use inwhich a sound signal is digitized and transmitted, since a data sequenceshows a random characteristic, a spectrum in the actual state of use hasa characteristic substantially the same as that at the time whenpseudo-random data is modulated. Therefore, under the worst condition,nonconformity may be allowed to some extent. An emission spectrum onlyhas to completely conform to an emission mask when the pseudo-randomdata is used.

Levels of respective peak spectra under the worst condition at the timewhen the P25-P1 modulation system is adopted are calculated from theBessel function. A degree of deviation of a spectrum from the mask D iscalculated.

The base band filter represented by Table 3 includes a raised cosinefilter and a shaping filter. A characteristic of this shaping filter isequal to a characteristic opposite to a characteristic of sinc(sin(x)/x)in which an amplitude is 1 at a frequency of 0 and is 0 at a symbolfrequency. Nominal frequency shifts in the symbols of +3 and −3 are +1.8kHz and −1.8 kHz, respectively. In the raised cosine filter, sinceintersymbol interference does not occur, an amplitude of a symbol at afilter output does not change. Therefore, frequency shifts at the timewhen the symbols of +3 and −3 are alternately sent are equal to thenominal frequency shifts when only the raised cosine filter is used.Since a frequency characteristic of the shaping filter is equal to acharacteristic opposite to that of sinc, the frequency characteristic isindicated by Expression (3) below. $\begin{matrix}{\begin{pmatrix}{{Characterictic}\quad{of}} \\{{the}\quad{shaping}\quad{fiter}}\end{pmatrix} = {\frac{1}{\sin\quad c} = \frac{f\quad{\pi/4.8}}{\sin\left( {f\quad{\pi/4.8}} \right)}}} & (3)\end{matrix}$

When the symbols of +3 and −3 are repeated, a frequency f is 2.4 kHz.When the frequency f is substituted in Expression (3), a characteristicof the shaping filter is π/2. Since an actual frequency shift changes bya degree equivalent to the characteristic of the shaping filter (=π/2)from the nominal frequency shifts, an actual frequency shift of thefilter including the raised cosine filter and the shaping filter is 1.8kHz×π/2=2.827 kHz. Values of the Bessel function at the time when amodulation frequency is set to 2.4 kHz and a frequency shift is set to2.827 kHz and values of the mask D calculated from Table 1 are shown inTable 6 below. TABLE 6 Standard Value Bessel Function Order Frequency ofMask D Value 1 2.4 kHz    0 dB  −6.15 dB 2 4.8 kHz    0 dB −16.25 dB 37.2 kHz −31.41 dB −30.11 dB 4 9.6 kHz −48.85 dB −46.60 dB 5 12.0 kHz −66.30 dB −65.07 dB

As shown in Table 6, in components of third order and higher orders,values of the Bessel function are below the standard by about 1 dB to 2dB. When the values of the Bessel function shown in Table 6 and thevalues of the respective peak spectra shown in FIG. 4 are compared,although there is a slight error, the values of the Bessel function andthe values of the peak spectra are substantially correlated. Therefore,since the frequency shift 0.822 kHz indicated by Expression (2) is afrequency shift completely conforming to the mask E under the worstcondition, if a slight fall below the standard is allowed under theworst condition as in the P25-P1 modulation system, a frequency shiftequal to or larger than 0.822 kHz may be used. Taking into account thefact that an error rate in the FSK modulation system such as the P25-P1modulation system depends on a frequency shift, it is advisable to setthe frequency shift as large as possible. Thus, a value of a spectrumhaving a frequency integer times as large as 1.2 kHz calculated from theBessel function and the standard value of the mask E are compared and,when an emission spectrum does not conform to the mask E under the worstcondition but pseudo-random data is used, the modulation index mfconforming to the mask E is 0.793 and a frequency shift takes a valueindicated by Expression (4) below.0.793×1.2=0.952 kHz   (4)

Values of the Bessel function at the time when a modulation frequency isset to 1.2 kHz and a frequency shift is set to 0.952 kHz and values ofthe mask E calculated from Table 4 are shown in Table 7 below. TABLE 7Standard Value Bessel Function Order Frequency of Mask E Value 1 1.2 kHz   0 dB  −8.72 dB 2 2.4 kHz    0 dB −22.54 dB 3 3.6 kHz −40.0 dB −40.00dB 4 4.8 kHz −65.0 dB −60.00 dB

The modulation index mf=0.793 is a modulation index at which a standardvalue of the mask E and a value of the Bessel function are the same inthe third-order component. In the fourth-order component, a value of theBessel function is below a standard value of the mask E by 5 dB. Asshown in Table 4, in a frequency band of fd>4.6 kHz, a standard value ofthe mask E changes according to a value of a transmission power P. Whenthe transmission power P is equal to or higher than 10 W, a standardvalue is −65 dB. When the transmission power P is 5 W, a standard valueis −62 dB. In general, in a portable radio apparatus, a transmissionpower is 5 W and, under this condition, a value of the Bessel functionis below a standard value of the mask E by 3 dB. When the fall below thestandard value of this degree is not significantly different from thefall below the standard with respect to the mask D in the case in whichthe P25-P1 modulation system is adopted shown in Table 6. Thus, it canbe said that the fall below the standard is within a tolerance as in thecase of the mask D.

In the above explanation, a maximum frequency shift for adapting anemission spectrum to the mask E at the time when the quaternary FSK isused at the transmission rate of 4800 bps is calculated. When digitaldata is modulated, it is a well-known fact that a waveform is shapedusing a base band filter. An error rate is affected by a base bandfilter used. As indicated by Expression (3), an actual frequency shiftoutputted from a modulator changes depending on a base band filter.Thus, nominal frequency shifts at the symbol levels of +3 and −3obtained by calculating backward the maximum frequency shift 0.952 kHzindicated by Expression (4) are different.

It is a general practice to use a Nyquist transmission path as atransmission path in order to hold down an error rate. Therefore, inthis embodiment, a Nyquist filter is used in a base band filer. An FMmodulator and an FM demodulator have transparency as long as distortiondoes not occur. Thus, in order to form a Nyquist transmission path, theNyquist filter only has to be arranged at a pre-stage of the FMmodulator or a post stage of the FM demodulator such that the base bandfilter has a Nyquist characteristic. Since the shaping filter and theintegrate and dump filter described in Table 3 have opposite frequencycharacteristics, it can be understood that, if the modulator and thedemodulator are integrated, only the raised cosine filter is left andthat the Nyquist transmission path is formed.

As described above, it is a necessary condition that a Nyquist filter isused in order to reduce an error rate. In addition, a nominal frequencyshift at which an actual frequency shift of 0.952 kHz is obtained isdifferent depending on a Nyquist filter used. Therefore, it is necessaryto select a Nyquist filter with which a largest nominal frequency shiftis obtained. In general, a filter having a raised cosine characteristicis used as a Nyquist filter. Combinations of filters in this case areshown in Table 8 below. TABLE 8 Combination Transmission Side ReceptionSide I Raised Cosine None II Raised Cosine + 1/ sinc sinc III NoneRaised Cosine IV sinc Raised Cosine + 1/sinc V Root Raised Cosine RootRaised Cosine VI Root Raised Cosine + 1/ Root Raised Cosine + sinc sincVII Root Raised Cosine + sinc Root Raised Cosine + 1/ sinc

Since a 1/sinc filter has a characteristic opposite to that of a sincfilter, the 1/sinc filter has a diverging frequency characteristic.Therefore, it is impossible to use the 1/sinc filter alone and acombination using only the 1/sinc filter is excluded. When no filter isprovided on the reception side, since band limitation is not applied tonoise and an S/N is deteriorated, an error rate extremely worsens. Whenno filter is provided on the transmission side, a rectangular wave isdirectly subjected to FM modulation. A spectrum expands infinitely anddoes not conform to the mask E. Consequently, among the combinations,the combination I and the combination III are excluded from objects.Since the combination IV only has the sinc filter on the transmissionside, band limitation is loose and spread of a spectrum is relativelylarge. Thus, the combination IV is also excluded from objects.Therefore, the four kinds of combinations II, V, VI, and VII are left ascandidates.

A nominal frequency shift for setting an actual maximum frequency shiftto 0.952 kHz when the candidate filters are used is calculated. A symbolrate is 2400 symbol/s and, in repetition of the symbols of +3 and −3, aspectrum is a sine wave of a frequency 1.2 kHz. When the raised cosinefilter is used, a nominal frequency shift and an actual frequency shiftare equal. When amplitudes in the respective filters at the frequency1.2 kHz are calculated with an amplitude in this case as a referencevalue, the amplitudes are calculated as indicated by Expressions (5),(6), and (7) below. $\begin{matrix}{\left( {{Amplitude}\quad{of}\quad{Raised}\quad{Cosine}} \right) = {0.5\quad\left( {{Reference}\quad{value}} \right)}} & (5) \\{\left( {{Amplitude}\quad{of}\quad{Root}\quad{Raised}\quad{Cosine}} \right) = \sqrt{0.5}} & (6) \\{\left( {{Amplitude}\quad{of}\quad\sin\quad c} \right) = {\frac{\sin\left( {\pi \times {1.2/2.4}} \right)}{\left( {\pi \times {1.2/2.4}} \right)} = \frac{2}{\pi}}} & (7)\end{matrix}$

From these expressions, amplitude scaling factors based on an amplitudein the raised cosine filter at 1.2 kHz of the four kinds of candidatefilters are indicated by Expressions (8), (9), (10), and (11) below.Candidate II: (Amplitude of Raised Cosine+1/sinc)=1/(2/π)=π/2   (8)Candidate V: (Amplitude of Root Raised Cosine)=√{square root over(0.5)}/0.5   (9)Candidate VI: (Amplitude of Root Raised Cosine+1/sinc)=√{square rootover (2)}×π/2=π/√{square root over (2)}  (10)Candidate VII :(Amplitude of Root Raised Cosine+sinc)=√{square root over(2)}2/π  (11)

In order to set an actual maximum frequency shift to 0.952 kHz, nominalfrequency shifts in the respective candidate filters are calculated asindicated by Expressions (12), (13), (14), and (15).Candidate II: (Shift of Raised Cosine+1/sinc)=0.952/(π/2)=0.6061 kHz  (12)Candidate V (Shift of Root Raised Cosine) 0.952/√{square root over(2)}=0.6732 kHz   (13)Candidate VI :(Shift of Root Raised Cosine+1 sinc)=0.952/(π/√{squareroot over (2)})=0.4286 kHz   (14)Candidate VII :(Shift of Root Raised Cosine+sinc)=0.952 /(√{square rootover (2)}×2/π)=1.0574 kHz   (15)

In the FSK modulation system, since an error rate is lower when afrequency shift is larger, a filter having a larger nominal frequencyshift shows a better characteristic. Considering the reception side, afilter with strong band limitation on noise has a better S/N and, as aresult, has a lower error rate. In the candidate VI, although bandlimitation on the reception side is strong, since a nominal frequencyshift is extremely small, an overall error rate is deteriorated. In thecandidate II, since a nominal frequency shift is the second lowest andband limit on the reception side is the loosest, an error rate is alsodeteriorated. A result obtained by calculating an error rate by applyingthe nominal frequency shifts of the filters calculated in Expressions(12) to (15) is shown in FIG. 10.

According to the result shown in FIG. 10, it is proved that the abovejudgment is correct. The best error rate is indicated when the base bandfilter of the candidate VII is used. The second best error rate isindicated when the base band filter of the candidate V is used. A C/N iscalculated from the result shown in FIG. 10 and an error rate withrespect to an input level of a receiver at the time when a noise indexof the receiver is assumed to be 7 dB is calculated and shown in FIG.11. When it is assumed that a reference sensitivity has an error rate of3%, in the candidate VII, −122 dBm is an input level at which thereference sensitivity is obtained. The FCC rule provides that areference sensitivity should be set to 12 dBSINAD when the analog FMmodulation system is used in the LMR system applicable to the channelspacing of 12.5 kHz. In a receiver having a noise index of 7 dB, aninput level at which a reception sensitivity of 12 dBSINAD can beobtained is about −120 dBm. When this reception sensitivity of 12dBSINAD and the candidate VII are compared, as shown in FIG. 11, thecandidate VII has a sensitivity better than the reception sensitivity byabout 2 dB. In the candidate V, the error rate of 3% is obtained by−120.5 dBm. The candidate V has a reference sensitivity equal to that inthe case in which the conventional analog FM modulation system is used.

From the above description, a minimum value of a frequency shift forcompletely adapting a range of actual frequency shifts that can be takento the mask E is 0.822 kHz indicated by Expression (2) and a maximumvalue thereof is 0.952 kHz indicated by Expression (4). It is possibleto obtain a modulation system conforming to the FCC rule to be enforcedin 2005 by calculating nominal frequency shifts of the candidates V andVII using Expressions (9) and (11). On the basis of these facts,embodiments will be explained.

First Embodiment

First, a transmission rate, a symbol rate, a base band filter, amodulation system, and a nominal frequency shift in the first embodimentare shown in Table 9 below. TABLE 9 Transmission Rate 4800 bps SymbolRate 2400 symbol/s Base Band Filter Transmission: A filter obtained bycombining a filter having a Root Raised Cosine characteristic with anarbitrary α and a filter having a sinc function characteristicReception: A filter obtained by combining a filter having a Root RaisedCosine characteristic with an arbitrary α and a filter having a 1/sincfunction characteristic Modulation System Quaternary FSK ModulationSystem Nominal Frequency Arbitrary in ranges +3 = +913 Hz to Shift +1057Hz and −3 = −913 Hz to −1057 Hz with respect to respective four symbollevels (±3, ±1) +1 and −1 are shifts ⅓ of +3 and −3, respectively

FIG. 12 is a block diagram showing a constitution of a ground mobilecommunication system 1 according the first embodiment of the invention.The ground mobile communication system 1 schematically includes atransmitting unit 10 and a receiving unit 20 and performs transmissionand reception of data at a transmission rate of 4800 bps.

In the ground mobile communication system 1, as a nominal frequencyshift, a predetermined value of ±913 kHz to ±1057 kHz is set withrespect to a symbol level ±3 and a value ⅓ of the predetermined value isset with respect to a symbol level ±1. This nominal frequency shift is afrequency that shifts when a symbol outputted from a mapper 12 describedlater is inputted to an FM modulator 14 without intervention of a baseband filter 13.

The transmitting unit 10 includes an encoder 11, the mapper 12, the baseband filter 13, and the FM modulator 14.

The encoder 11 applies predetermined format processing such as encodingprocessing, error correction code addition processing, and synchronouscode addition processing to sound data, character data, and the like togenerate a binary signal and supplies the binary signal generated to themapper 12.

The mapper 12 sequentially converts a binary signal sequentiallyinputted from the encoder 11 into quaternary symbols (±3, ±1) two bitsat a time and supplies the quaternary symbols to the base band filter13. This symbol is a rectangular voltage signal having a width of apredetermined symbol time. In this embodiment, since a transmission rateis 4800 bps, a symbol rate is 2400 symbol/s and a frequency of a symbolsequence at the time when symbols of +3 and −3 are alternately outputtedfrom the mapper 12 is 1.2 kHz.

The base band filter 13 includes a Nyquist filter. The base band filter13 blocks a predetermined frequency component of a symbol inputted fromthe mapper 12 and outputs a waveform signal. In this embodiment, thebase band filter 13 includes a root raised cosine filter 131 and a sincfilter 132.

When a frequency of an inputted rectangular signal is 1.2 kHz, the rootraised cosine filter 131 outputs a waveform signal having an amplitude√{square root over (2)} times as large as that of this rectangularsignal. When a frequency of an inputted rectangular signal is 1.2 kHz,the sinc filter 132 outputs a waveform signal having an amplitude 2/πtimes as large as that of this rectangular signal.

In other words, the base band filter 13 in this embodiment shapes arectangular symbol with a frequency of 1.2 kHz inputted from the mapper12 into a waveform signal having an amplitude 2√{square root over (2)}/πtimes as large as that of the symbol and outputs the waveform signal.

The FM modulator 14 shifts a frequency of a signal transmitted to thereceiving unit 20 according to a magnitude of an amplitude of a waveformsignal outputted from the base band filter 13 to thereby modulate thefrequency (FM modulation).

To explain the above more in detail, when a level of a symbol outputtedfrom the mapper 12 is +3, the FM modulator 14 shifts a frequency of asignal to be transmitted by +Δf(+0.822 kHz≦Δf≦+0.952 kHz). When a symbollevel is −3, the FM modulator 14 shifts a frequency of a signal to betransmitted by −Δf(−0.822 kHz≦Δf≦−0.952 kHz). When a symbol level is +1,the FM modulator 14 shifts a frequency of a signal to be transmitted by+Δf/3. When a symbol level is −1, the FM modulator 14 shifts a frequencyof a signal to be transmitted by −Δf.

The FM modulator 14 radiates the signal subjected to FM modulation intothe air via a not-shown antenna and transmits the signal to thereceiving unit 20.

The receiving unit 20 includes an FM demodulator 21, a base band filter22, a demapper 23, and a decoder 24.

The FM demodulator 21 demodulates a signal inputted and received via anot-shown antenna and supplies a quaternary signal obtained bydemodulating the signal to the base band filter 22.

The base band filter 22 includes a root raised cosine filter 131 and a1/sinc filter 231 having a characteristic opposite to that of the sincfilter 132. The base band filter 22 blocks a predetermined frequencycomponent of a quaternary signal inputted from the FM demodulator 21 andoutputs a quaternary signal having an amplitude π/2√{square root over(2)} times as large as that of the input signal.

A Nyquist transmission path is formed in the ground mobile communicationsystem 1 by the base band filter 22 and the base band filter 13 of thetransmitting unit 10.

The demapper 23 sequentially converts a quaternary signal inputted fromthe base band filter 22 into a binary signal of 2 bits and supplies thebinary signal converted to the decoder 24.

The decoder 24 applies decoding processing, error correction processing,and the like to a binary signal supplied from the demapper 23 andoutputs sound data, character data, and the like transmitted from thetransmitting unit 10.

A communication operation of the ground mobile communication system 1having the constitution described above will be explained with referenceto a flowchart shown in FIG. 13.

When sound data, character data, and the like are inputted to theencoder 11 of the transmitting unit 10, the ground mobile communicationsystem 1 starts the communication operation shown in the flowchart inFIG. 13.

The encoder 11 applies encoding processing to the sound data, thecharacter data, and the like inputted to generate a binary signal andoutputs the binary signal generated to the mapper 12 (step S101).

The mapper 12 applies mapping processing to the binary signal inputtedfrom the encoder 11 to sequentially convert the binary signal intoquaternary symbols two bits at a time and outputs the symbols convertedto the base band filter 13 (step S102).

The base band filter 13 shapes a waveform signal by applying bandlimitation processing to the symbols inputted from the mapper 12 toblock a predetermined frequency component and outputs the waveformsignal shaped to the FM modulator 14 (step S103).

The FM modulator 14 applies FM modulation signal to a transmissionsignal according to a magnitude of an amplitude of the waveform signaloutputted from the base band filter 13 (step S104) and radiates thesignal subjected to FM modulation to the air via the not-shown antennato thereby transmit the sound data, the character data, and the like tothe receiving unit 20.

The FM demodulator 21 of the receiving unit 20 applies FM demodulationprocessing to the signal inputted via the not-shown antenna and receivedand outputs a quaternary signal obtained by demodulating the signal tothe base band filter 22 (step S201).

The base band filter 22 applies band limitation processing to thequaternary signal inputted from the FM demodulator 21 and outputs thequaternary signal with a predetermined frequency component blocked tothe demapper 23 (step S202).

The demapper 23 applies demapping processing to the quaternary signalinputted from the base band filter 22, converts the quaternary signalinto a binary signal of 2 bits, and outputs the binary signal convertedto the decoder 24 (step S203).

The decoder 24 applies decoding processing and the like to the binarysignal supplied from the demapper 23 (step S204) to decode the sounddata, the character data, and the like transmitted from the transmittingunit 10 and outputs the sound data, the character data, and the like.

A waveform of an emission spectrum at the time when a nominal frequencyshift is set to 1.057 kHz and at the time of the worst condition (whendata corresponding to the symbol +3 and data corresponding to the symbol−3 are alternately inputted to the transmitting unit 10) is shown inFIG. 14. An emission spectrum at the time when pseudo-random data isinputted to the transmitting unit 10 is shown in FIG. 15.

Second Embodiment

A second embodiment will be explained. A transmission speed, a symbolrate, a base band filter, a modulation system, and a nominal frequencyshift in the second embodiment are shown in Table 10 below. TABLE 10Transmission Rate 4800 bps Symbol Rate 2400 symbol/s Base Band FilterTransmission: A filter having a Root Raised Cosine characteristic withan arbitrary α Reception: A filter having a Root Raised Cosinecharacteristic with an arbitrary α Modulation System Quaternary FSKModulation System Nominal Frequency Arbitrary in ranges +3 = +581 Hz toShift +673 Hz and −3 = −581 Hz to −673 Hz with respect to respectivefour symbol levels (±3, ±1) +1 and −1 are shifts ⅓ of +3 and −3,respectively

FIG. 16 is a block diagram showing a constitution of a ground mobilecommunication system 2 according to the second embodiment of theinvention. The ground mobile communication system 2 schematicallyincludes a transmitting unit 30 and a receiving unit 40. Componentsidentical with those in the first embodiment are denoted by theidentical reference numerals. Explanations of the components areomitted.

In the ground mobile communication system 2, as a nominal frequencyshift, a predetermined value of ±581 kHz to ±673 kHz are set withrespect to a symbol level ±3 and a value ⅓ of the predetermined value isset with respect to a symbol level ±1.

The transmitting unit 30 includes the encoder 11, the mapper 12, a baseband filter 33, and the FM modulator 14. The base band filter 33includes a root raised cosine filter 131. The base band filter 33 shapesa rectangular symbol with a frequency of 1.2 kHz inputted from themapper 12 into a waveform signal having an amplitude √{square root over(2)} times as large as that of the symbol and outputs the waveformsignal.

The receiving unit 40 includes the FM demodulator 21, a base band filter42, the demapper 23, and the decoder 24. The base band filter 42includes a root raised cosine filter 131. The base band filter 42 blocksa predetermined frequency component of a quaternary signal inputted fromthe FM demodulator 21 and outputs a quaternary signal having anamplitude √{square root over (2)} times as large as that of the inputsignal.

As in the ground mobile communication system 1, a Nyquist transmissionpath is formed in the ground mobile communication system 2 by the baseband filter 42 and the base band filter 33 of the transmitting unit 30.

A waveform of an emission spectrum at the time when a nominal frequencyshift is set to 0.673 kHz and at the time of the worst condition (whendata corresponding to the symbol +3 and data corresponding to the symbol−3 are alternately inputted to the transmitting unit 30) is shown inFIG. 17. An emission spectrum at the time when pseudo-random data isinputted to the transmitting unit 30 is shown in FIG. 18.

From the first and second embodiments, in the ground mobilecommunication systems 1 and 2, even if nominal frequency shifts set aredifferent, frequency shifts of signals actually outputted from thetransmitting units 10 and 30 are the same. Thus, the emission spectrashown in FIGS. 14 and 17 indicate substantially the samecharacteristics. Since it is possible to confirm that a level of aspectrum is substantially equal to a level calculated from the Besselfunction shown in Table 7, it is possible to prove correctness ofinstallation of the ground mobile communication systems 1 and 2.Moreover, as shown in FIGS. 15 and 18, when pseudo-random data is used,since an emission spectrum has a sufficient room with respect to thestandard of the mask E, the ground mobile communication systems 1 and 2are capable of conforming to the FCC rule to be enforced in 2005.

According to the above description, the quaternary FSK modulation systemis applied to the LMR system in this embodiment. Thus, it is possible todirectly apply a nonlinear power amplifier and an FM modulation anddemodulation circuit of a radio apparatus of analog FM modulationpresently operated in the LMR system applicable to the 12.5 kHz channelspacing to the LMR system in this embodiment. Therefore, the LMR systemin this embodiment can conform to the FCC rule to be enforced in 2005without using a linear power amplifier having a problem in terms ofcost.

It is possible to handle sound data, character data, and the like alltogether by applying a modulation system that uses digital data. Thus,the LMR system is suitable for demand for data communication in thesedays.

Since it is possible to reduce an error rate while adapting the LMRsystem to the FCC rule to be enforced in 2005, it is possible to attaina sensitivity equal to or higher than a reference sensitivity of theconventional analog FM modulation. A call distance is longer comparedwith the conventional call distance. Moreover, since a transmissionpower falls when the call distance is the same, it is possible to reducea consumed current in the LMR system.

In order to keep downward compatibility, a radio apparatus actually usedis required to also implement an operation mode of analog FM modulationused in the LMR system applicable to the 12.5 kHz channel spacing.However, in the LMR system in this embodiment, since the quaternary FSKmodulation system having compatibility with a large number of circuitsis adopted, dual mode design is possible. Moreover, the P25-P1modulation system is based on the quaternary FSK modulation system,although there is a difference in parameters. Thus, dual mode design incombination with the P25-P1 modulation system is also possible.

The invention is not limited to the embodiments and variousmodifications and applications of the embodiments are possible.Modifications of the embodiments applicable to the invention will beexplained.

In the embodiments, the base band filter of the candidate VII is used inthe first embodiment and the base band filter of the candidate V is usedin the second embodiment. However, the invention is not limited to thesebase band filters. The base band filter of the candidate II may be used.

A transmission rate, a symbol rate, a base band filter, a modulationsystem, and a nominal frequency shift in the case in which the base bandfilter of the candidate II is used are shown in Table 11 below. TABLE 11Transmission Rate 4800 bps Symbol Rate 2400 symbol/s Base Band FilterTransmission: A filter obtained by combining a filter having a RaisedCosine characteristic with an arbitrary α and a filer having a 1/sincfunction characteristic Reception: A filter having a sinc functioncharacteristic Modulation System Quaternary FSK Modulation SystemNominal Frequency Arbitrary in ranges of +3 = +523 Hz Shift to +606 Hzand −3 = −523 Hz to −606 Hz with respect to respective four symbollevels (±3, ±1) +1 and −1 are shifts ⅓ of +3 and −3, respectively

FIG. 19 is a block diagram showing a constitution of a ground mobilecommunication system 3 according to a modification of the invention. Theground mobile communication system 3 schematically includes atransmitting unit 50 and a receiving unit 60. Components identical withthose in the first and second embodiments are denoted by the identicalreference numerals and explanations of the components are omitted.

In the ground mobile communication system 3, as a nominal frequencyshift, a predetermined value of ±0.523 kHz to ±0.606 kHz are set withrespect to a symbol level ±3 and a value ⅓ of the predetermined value isset with respect to a symbol level ±1.

The transmitting unit 50 includes the encoder 11, the mapper 12, a baseband filter 53, and the FM modulator 14. The base band filter 53includes a root raised cosine filter 531 and a 1/sinc filter 231. Thebase band filter 53 shapes a rectangular symbol with a frequency of 1.2kHz inputted from the mapper 12 into a waveform signal having anamplitude π/2 times as large as that of the symbol and outputs thewaveform signal.

The receiving unit 60 includes the FM demodulator 21, a base band filter62, the demapper 23, and the decoder 24. The base band filter 62includes a sinc filter 132 having a characteristic opposite to that ofthe 1/sinc filter 231. The base band filter 62 blocks a predeterminedfrequency component of a quaternary signal inputted from the FMdemodulator 21 and outputs a quaternary signal having an amplitude 2/πtimes as large as that of the input signal.

As in the ground mobile communication systems 1 and 2, a Nyquisttransmission path is formed in the ground mobile communication system 3by the base band filter 62 and the base band filter 53 of thetransmitting unit 50 described above.

As described above, the base band filter of the candidate II is inferiorto the base band filters of the candidates VII and V in terms of asensitivity but includes the same filter constitution as the base bandfilter used in the P25-P1 modulation system. Therefore, it is possibleto use a coefficient of a filter that is the same as the base bandfilter used in the P25-P1 modulation system as long as a roll-off factora has the same value as that of the base band filter used in the P25-P1modulation system. Thus, it is possible to reduce a circuit size or amemory size.

In the embodiments, the quaternary FSK modulation system is adopted as amulti-value FSK modulation system. However, the invention is not limitedto this. An octonary FSK modulation system, a hexadecimal FSK modulationsystem, and the like may be adopted. For example, when the octonary FSKmodulation system is used, since a symbol rate is fixed at 2400symbol/s, a transmission rate is 7200 bps. A symbol level takes eightvalues of ±7, ±5, ±3, and ±1. When symbols of +7 and −7 are alternatelysent, a sine wave of 1.2 kHz is formed. Therefore, if actual frequencyshifts in the symbols of +7 and −7 are set to, for example, the valuesdefined in the first and second embodiments, the modification and thelike, it is possible to adapt an emission spectrum to the mask E as inthe case in which a symbol level takes four values. By adopting such acondition, it is possible to obtain a higher transmission rate by using,other than the octonary FSK modulation system, the hexadecimal FSKmodulation system or an FSK modulation system of a larger value whileadapting an emission spectrum to the mask E.

In the embodiments, the mobile communication system is the ground mobilecommunication system. However, the invention is not limited to this. Themobile communication system may be a mobile communication system used onthe sea.

INDUSTRIAL APPLICABILITY

According to the invention, it is possible to provide a modulatingapparatus, a ground mobile communication system, a modulating method,and a communication method that can conform to the FCC rule to beenforced in 2005 without using a linear power amplifier.

1. A modulating apparatus in a mobile communication system that performsdata communication at a rate for transmitting 2400 multi-value symbolsper second, characterized by comprising: a base band filter that blocksan unnecessary frequency component of a multi-value symbol inputted andoutputs a waveform signal; and frequency shifting and modulating meansfor shifting to modulate a frequency of an output signal according to amagnitude of an amplitude of the waveform signal inputted from the baseband filter, and in that the frequency shifting and modulating means isadjusted such that, when a symbol having a maximum absolute value isinputted, an output signal has an absolute value of a frequency shift ina range of 0.822[kHz] to 0.952[kHz].
 2. A modulating apparatus in amobile communication system that performs data communication at atransmission rate of 2400×(n+1) (n: natural number) [bps], characterizedby comprising: symbol converting means for sequentially converting abinary signal generated by encoding predetermined data into a2^((n+1))-ary symbol, which includes (2^((n+1))+1−2k) (1≦k≦2^((n+1)))values, (n+1) bits at a time and outputting the symbol; a base bandfilter that blocks an unnecessary frequency component of a symbolinputted from the symbol converting means and outputs a waveform signal;and frequency shifting and modulating means for shifting to modulate afrequency of an output signal according to a magnitude of an amplitudeof the waveform signal inputted from the base band filter, and in thatwhen a symbol of ±(2^((n+1))−1) is outputted from the symbol convertingmeans, a frequency shift of the output signal from the frequencyshifting and modulating means is set to take a value in a range of±0.822[kHz] to ±0.952[kHz].
 3. The modulating apparatus according toclaim 1 or 2, characterized in that the base band filter is a Nyquistfilter.
 4. A mobile communication system comprising: a transmitter thatperforms transmission of data at a transmission rate of 2400×(n+1) (n:natural number) [bps]; and a receiver that receives data transmittedfrom the transmitter, characterized in that the transmitter includes:encoding means for encoding predetermined data to generate a binarysignal; symbol converting means for sequentially converting a binarysignal generated by the encoding means into a 2^((n+1))-ary symbol,which includes (2^((n+1))+1−2k) (1≦k≦2^((n+1))) values, (n+1) bits at atime and outputting the symbol; a first base band filter that blocks anunnecessary frequency component of a symbol inputted from the symbolconverting means and outputs a waveform signal; and frequency shiftingand modulating (FM) means for transmitting a signal, which is obtainedby shifting to modulate a frequency according to a magnitude of anamplitude of the waveform signal inputted from the first base bandfilter, to the receiver, the receiver includes: demodulating means fordemodulating the signal transmitted from the transmitter and receivedand outputting a 2^((n+1))-ary signal; a second base band filter thatblocks an unnecessary frequency component of the 2^((n+1))-ary signaloutputted from the modulating means and outputs the 2^((n+1))-arysignal; binary signal converting means for sequentially converting a2^((n+1))-ary signal inputted from the second base band filter into abinary signal of (n+1) bits and outputting the binary signal; anddecoding means for decoding a binary signal inputted from the binarysignal generating means and outputting the predetermined data, and whena symbol of ±(2^((n+1))−1) is outputted from the symbol convertingmeans, a frequency shift of a signal outputted from the frequencyshifting and modulating means is set in a range of ±0.822[kHz] to±0.952[kHz].
 5. The mobile communication system according to claim 4,characterized in that the first and second base band filters are Nyquistfilters.
 6. The mobile communication system according to claim 4 or 5,characterized in that the first base band filter includes a root raisedcosine filter and a sinc filter, the second base band filter includes aroot raised cosine filter and a 1/sinc filter that has a characteristicopposite to that of the sinc filter, and a nominal frequency shift ofthe symbol of ±(2^((n+1))−1) is set to a value π/2√{square root over(2)} times as large as a frequency shift of a signal outputted from thefrequency shifting and modulating means.
 7. The mobile communicationsystem according to claim 4 or 5, characterized in that the first andsecond base band filters include root raised cosine filters, and thenominal frequency shift of the symbol of ±(2^((n+1))−1) is set to avalue 1/√{square root over (2)} times as large as a frequency shift of asignal outputted from the frequency shifting and modulating means. 8.The mobile communication system according to claim 4 or 5, characterizedin that the first base band filter includes a raised cosine filter and a1/sinc filter, the second base band filter includes a sinc filter thathas a characteristic opposite to that of the 1/sinc filter, and thenominal frequency shift of the symbol of ±(2^((n+1))−1) is set to avalue 2/π times as large as a frequency shift of a signal outputted fromthe frequency shifting and modulating means.
 9. A modulating method in amobile communication system that performs data communication at a ratefor transmitting 2400 multi-value symbols per second, characterized bycomprising: a step of blocking an unnecessary frequency component of amulti-value symbol inputted and outputting a waveform signal; and afrequency shifting and modulating step of shifting to modulate afrequency of an output signal according to a magnitude of an amplitudeof the waveform signal inputted, and in that in the frequency shiftingand modulating step, signal processing is performed such that, when asymbol having a maximum absolute value is inputted, an output signal hasan absolute value of a frequency shift in a range of 0.822[kHz] to0.952[kHz].
 10. A modulating method in a mobile communication systemthat performs data communication at a transmission rate of 2400×(n+1)(n: natural number) [bps], characterized by comprising: a symbolconverting step of sequentially converting a binary signal generated byencoding predetermined data into a 2^((n+1))-ary symbol, which includes(2^((n+1))+1−2k) (1≦k≦2 values, (n+1) bits at a time and outputting thesymbol; a step of blocking an unnecessary frequency component of asymbol inputted from the symbol converting means and outputting awaveform signal; and a frequency shifting and modulating step ofshifting to modulate a frequency of an output signal according to amagnitude of an amplitude of the waveform signal inputted, and in thatwhen a symbol of ±(2^((n+1))−1) is outputted from the symbol convertingstep, a frequency shift of the output signal from the frequency shiftingand modulating step is set in a range of ±0.822[kHz] to ±0.952[kHz]. 11.A communication method in a mobile communication system including atransmitter that performs transmission of data at a transmission rate of2400×(n+1) (n: natural number) [bps] and a receiver that receives datatransmitted from the transmitter, characterized by comprising: anencoding step of encoding predetermined data to generate a binarysignal; a symbol converting step of sequentially converting a binarysignal generated by the encoding step into a 2^((n+1))-ary symbol, whichincludes (2^((n+1))+1−2k) (1≦k≦2^((n+1))) values, (n+1) bits at a timeand outputting the symbol; a step of blocking an unnecessary frequencycomponent of a symbol inputted from the symbol converting step andoutputting a waveform signal; a frequency shifting and modulating stepof transmitting a signal, which is obtained by shifting to modulate afrequency according to a magnitude of an amplitude of the waveformsignal inputted from the first base band filter, to the receiver; ademodulating step of demodulating the signal transmitted from thetransmitter and received and outputting a 2^((n+1))-ary signal; a stepof blocking an unnecessary frequency component of the 2^((n+1))-arysignal outputted from the modulating step and outputting the2^((n+1))-ary signal; a binary signal converting step of sequentiallyconverting a 2^((n+1))-ary signal inputted into a binary signal of (n+1)bits and outputting the binary signal; and a decoding step of decoding abinary signal inputted from the binary signal generating step andoutputting the predetermined data, and in that when a symbol of±(2^((n+1))−1) is outputted from the symbol converting step, a frequencyshift of a signal outputted from the frequency shifting and modulatingstep is set to take a value in a range of ±0.822[kHz] to ±0.952[kHz].